java - 使用 Firebase 数据库获取每个子项的最后一个子项

Question

我想获取下图中的 sender_uid 值:

这是我下面的代码。我没有得到任何值(value)，我也不知道为什么。有人可以解释一下代码有什么问题吗？谢谢!

编辑:我只想要每个家长的最后 sender_uid - 例如:

• 对于节点391rfBEXjVUplEW8aoahMtluSmz1_6dTv7oHnWOh55XzRuf9ouZFZkGt2
  我只需要来自 LJxvDjdetWz1-DAFVCy 的 sender_uid，不需要来自 LJxvA65WA7MX8aaX5nF 的消息。

  mDatabase.addValueEventListener(new ValueEventListener() {
          @Override
          public void onDataChange(DataSnapshot dataSnapshot) {
              List<UsersListGetter> usersList = new ArrayList<>();
              recyclerView.setAdapter(new UsersListAdapter(usersList, getApplicationContext()));
  
              recyclerView.setAdapter(new UsersListAdapter(null, getApplicationContext()));
  
              for (DataSnapshot ch : dataSnapshot.getChildren()) {
                  String senderReceiver[] = ch.getKey().split("_");
  
                  if (senderReceiver[0].equals(mAuth.getUid()) || senderReceiver[1].equals(mAuth.getUid())) {
                      Query lastQuery = mDatabase.child(ch.getKey()).orderByKey().limitToLast(1);
                      lastQuery.addListenerForSingleValueEvent(new ValueEventListener() {
                          @Override
                          public void onDataChange(DataSnapshot dataSnapshot) {
                              String message = dataSnapshot.child("message_body").getValue().toString();
                              Log.d("MSG", message);
                          }
  
                          @Override
                          public void onCancelled(DatabaseError databaseError) {
                              //Handle possible errors.
                          }
                      });
                  }
              }
          }
  
      @Override
      public void onCancelled(@NonNull DatabaseError databaseError) {
  
      }
  });
  

Answer 1

您可以通过这种方式从每个节点获取sender_uid

首先创建一个pojo类来获取你想要的数据

public class UserPojo {

    private String sender_uid;

    public UserPojo() {

    }

    public String getSender_uid() {
        return sender_uid;
    }

    public void setSender_uid(String sender_uid) {
        this.sender_uid = sender_uid;
    }
}

然后为每个聊天引用检索它

mDatabase.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                List<UsersListGetter> usersList = new ArrayList<>();
                recyclerView.setAdapter(new UsersListAdapter(usersList, getApplicationContext()));

                recyclerView.setAdapter(new UsersListAdapter(null, getApplicationContext()));

                for (DataSnapshot ch : dataSnapshot.getChildren()) {
                  UserPojo up = ch.getValue(UserPojo.class);
                  String senderUid = up.getSender_uid(); //here you got your sender_uid

                        Query lastQuery = mDatabase.child(ch.getKey()).orderByKey().limitToLast(1);
                        lastQuery.addListenerForSingleValueEvent(new ValueEventListener() {
                            @Override
                            public void onDataChange(DataSnapshot dataSnapshot) {
                                String message = dataSnapshot.child("message_body").getValue().toString();
                                Log.d("MSG", message);
                            }

                            @Override
                            public void onCancelled(DatabaseError databaseError) {
                                //Handle possible errors.
                            }
                        });

                }
            }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });